Paging and segmentation
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This document summarizes and compares paging and segmentation, two common memory management techniques. Paging divides physical memory into fixed-size frames and logical memory into same-sized pages. It maps pages to frames using a page table. Segmentation divides logical memory into variable-sized segments and uses a segment table to map segment numbers to physical addresses. Paging avoids external fragmentation but can cause internal fragmentation, while segmentation avoids internal fragmentation but can cause external fragmentation. Both approaches separate logical and physical address spaces but represent different models of how a process views memory.
![ User View of logical memory
◦ Linear array of bytes
Reflected by the ‘Paging’ memory
scheme
◦ A collection of variable-sized
entities
User thinks in terms of “subroutines”,
“stack”, “symbol table”, “main program”
which are somehow located somewhere
in memory.]
Segmentation supports this user
view. The logical address space
is a collection of segments.](https://image.slidesharecdn.com/pagingandsegmentationa005a006-140520095359-phpapp02/85/Paging-and-segmentation-20-320.jpg)
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Paging and segmentation
- 2. Paging Presenter Kosha Raval Segmentation Presenter Senaea Ukaji
- 3. Physical memory is divided into fixed size- blocks called FRAMES Logical memory is divided into blocks of the same size called PAGES A frame has the same size as a page is a place where a (logical) page can be (physically) placed Memory-management technique that permits the physical address space of a process to be non- contiguous
- 6. Every address generated by the CPU is divided into two parts: Page number (p) and Page offset (d)
- 7. The page number is used as an index into a Page Table The page size is defined by the hardware The size of a page is typically a power of 2, varying between 512 bytes and 16MB per page Reason: If the size of logical address is 2^m and page size is 2^n, then the high-order m-n bits of a logical address designate the page number Paging Hardware
- 9. When we use a paging scheme, we have no external fragmentation: ANY free frame can be allocated to a process that needs it. However, we may have internal fragmentation If the process requires n pages, at least n frames are required The first page of the process is loaded into the first frame listed on free-frame list, and the frame number is put into page table Paging Example
- 10. Paging Example
- 11. Paging Example
- 12. To implement paging, the simplest method is to implement the page table as a set of registers However, the size of register is limited and the size of page table is usually large Therefore, the page table is kept in main memory
- 14. If we want to access any location, we must first index into the page table This requires atleast one memory access The standard solution is to use a special, small, fast cache, called Translation look-aside buffer (TLB) or associative memory
- 16. If the page number is not in the TLB (TLB miss) a memory reference to the page table must be made. In addition, we add the page number and frame number into TLB If the TLB already full, the OS have to must select one for replacement Some TLBs allow entries to be wire down, meaning that they cannot be removed from the TLB, for example kernel codes The percentage of times that a particular page number is found in the TLN is called hit ratio If it takes 20 nanosecond to search the TLB and 100 nanosecond to access memory
- 17. ADVANTAGES No external Fragmentation Simple memory management algorithm Swapping is easy (Equal sized Pages and Page Frames) Share common code especially in a time-sharing environment DISADVANTAGES Internal fragmentation Page tables may consume more memory. Multi level paging leads to memory reference overhead.
- 18. There is another way in which addressable memory can be subdivided, known as segmentation
- 19. An important part of the memory management is that become unavoidable with paging is the separation of the user’s view of the memory and the actual physical memory The users view is mapped to the physical memory Thus the differentiation comes between the logical and physical memory. Do users view memory as linear array of bytes some containing instruction and other containing data??? No…Rather they would see it as collection of segments…….
- 20. User View of logical memory ◦ Linear array of bytes Reflected by the ‘Paging’ memory scheme ◦ A collection of variable-sized entities User thinks in terms of “subroutines”, “stack”, “symbol table”, “main program” which are somehow located somewhere in memory.] Segmentation supports this user view. The logical address space is a collection of segments.
- 21. Although the user can refer to objects in the program by a two-dimensional address, the actual physical address is still a one-dimensional sequence Thus, we need to map the segment number This mapping is effected by a segment table In order to protect the memory space, each entry in segment table has a segment base and a segment limit
- 22. Logical Address space Segment number Offset The mapping of the logical address to the physical address is done with the help of the segment table. Segment Limit Segment Base Other bits A bit is needed to determine if the segment is already in main memory (P) Another bit is needed to determine if the segment has been modified since it was loaded in main memory (M) the length of the segment SEGMENT TABLE starting address of the corresponding segment in main memory
- 23. Segments are variable-sized ◦ Dynamic memory allocation required (first fit, best fit, worst fit). External fragmentation ◦ In the worst case the largest hole may not be large enough to fit in a new segment. Note that paging has no external fragmentation problem. Each process has its own segment table ◦ like with paging where each process has its own page table. The size of the segment table is determined by the number of segments, whereas the size of the page table depends on the total amount of memory occupied. Segment table located in main memory ◦ as is the page table with paging Segment table base register (STBR) ◦ points to current segment table in memory Segment table length register (STLR) ◦ indicates number of segments
- 26. Segmentation lends itself to the implementation of protection and sharing policies Each entry has a base address and length so inadvertent memory access can be controlled Sharing can be achieved by segments referencing multiple processes Two processes that need to share access to a single segment would have the same segment name and address in their segment tables.
- 27. No internal fragmentation Segment tables consume less memory than page tables ( only one entry per actual segment as opposed to one entry per page in Paging method) Because of the small segment table, memory reference is easy Lends itself to sharing data among processes. Lends itself to protection. As the individual lines of a page do not form one logical unit, it is not possible to set a particular access right to a page. Note that each segment could be set up an access right
- 28. External fragmentation. Costly memory management algorithm Unequal size of segments is not good in the case of swapping.
- 29. With paging physical memory is divided into fixed- size frames. When memory space is needed, as many free frames are occupied as necessary. These frames can be located anywhere in memory, the user process always sees a logical contiguous address space With segmentation the memory is not systematically divided. When a program needs k segments (usually these have different sizes), the OS tries to place these segments in the available memory holes. The segments can be scattered around memory. The user process does not see a contiguous address space, but sees a collection of segments (of course each individual segment is contiguous as is each page or frame).
- 31. Paging Segmentation Each process is assigned its page table. Page table size proportional to allocated memory Often large page tables and/or multi-level paging Internal fragmentation Free memory is quickly allocated to a process Each process is assigned a segment table Segment table size proportional to number of segments Usually small segment tables External fragmentation. Lengthy search times when allocating memory to a process.
- 32. Thank You
Editor's Notes
- #14: explAINATION From handout pdf slide 11 12 13